1. Field of the Invention
The present invention relates to an image forming apparatus which provides an image which is visualized as a result of a plurality of colors being overlaid together, in an electrophotographic printer or such. In particular, the present invention relates to an art of forming misregistration correction patterns, and detecting the thus-formed misregistration correction patterns, to carry out misregistration correction.
2. Description of the Related Art
In a tandem-type image forming apparatus, toner images of respective colors (for example, respective expression colors such as black (BK), cyan (C), magenta (M) and yellow (Y)) are formed by different image forming parts. The toner images are then overlaid together and a color image is formed.
In such an image forming apparatus, a stable color image may not be obtained (i.e., so-called color shifts may occur) when positions (i.e., image formed positions) of toner images of respective colors being overlaid together shift from each other. Therefore, in an image forming apparatus in the related art, a misregistration correction pattern is formed for each color, a pattern detecting part is used to detect a position of the misregistration correction pattern of each color, and misregistration correction is performed. In this image forming apparatus, an adjustment is carried out in such a manner that the misregistration patterns of the respective colors may be overlaid at the same position. As a result, in the image forming apparatus in the related art, color shifts caused by misregistration can be reduced, and thus a stable color image (i.e., a high-quality color image) can be formed.
However, in the image forming apparatus in the related art, after a certain time has elapsed from a time of misregistration correction being carried out, misregistration may occur again for various reasons. Therefore, in the image forming apparatus in the related art, misregistration correction should be carried out periodically.
It is noted that, a certain amount of toner is consumed for misregistration correction other than image forming operation which his actually desired by a user.
Causes of misregistration may be, for example, a change in a photosensitive characteristic, a change in an operation environment, and in particular, a change in an electrostatic characteristic. Therefore, a frequency of carrying out misregistration correction depends on a change of conditions such as an operating condition of an image forming apparatus, an installation condition of the image forming apparatus, or such. Therefore, in some cases, misregistration correction may be carried out frequently, and thereby, toner consumption may increase, which causes a disadvantage for a user.
Further, when a printing request is given during misregistration correction being carried out, an extra time is required until the required printing is finished. When misregistration correction is carried out frequently, a time required for carrying out the misregistration correction may constitute a down time, and may adversely affect the usability of the image forming apparatus. Further, such a down time may occur also when power supply is started in the image forming apparatus, and thereby, a time delay may be required before the required printing can be started.
In contrast thereto, Japanese Laid-Open Patent Application No. 2002-162805 for example discusses, an image forming apparatus in which, when misregistration correction for a sub-scan direction and a main scan direction is carried out, the number of misregistration correction patterns to be formed for each color for each scan direction is made different, a misregistration amount is calculated based on a position of each pattern, and correction is carried out. In this image forming apparatus, correction is regularly carried out based on the detection results of misregistration correction pattern positions for the entire length of a conveyance belt or an intermediate transfer belt, whereas, when continuous printing is carried out, correction is carried out based on a predetermined number of pattern positions.
Japanese Laid-Open Patent Application No. 2006-215524 discusses an image forming apparatus in which, as misregistration correction pattern image data, fine adjustment patterns (i.e., patterns used for fine correction) and coarse adjustment patterns (i.e., patterns for rough correction) are provided separately. Then, according to a condition at a time of correction, either type of the patterns is used for misregistration correction. That is, in the image forming apparatus, when a misregistration amount beyond a range of fine adjustment is calculated (i.e., a calculated misregistration amount is so large), misregistration correction patterns used for misregistration correction are changed from the fine adjustment patterns into the coarse adjustment patterns.
However, in these image forming apparatuses in the related art, the above-mentioned problems may not be completely solved.